A known type of a fuel cell is a solid oxide fuel cell (hereinafter may be referred to as “SOFC”). A fuel cell electricity generation unit (hereinafter may be referred to simply as an “electricity generation unit”), which is the smallest unit of electricity generation of an SOFC, includes a single cell, a metal separator, first and second interconnectors, a metal frame member, and a gas sealing member. The single cell includes an electrolyte layer, a cathode, and an anode such that the cathode and the anode face each other in a predetermined direction (hereinafter referred to as “first direction”) and the electrolyte layer intervenes between these electrodes. The separator is a frame member having a through hole, and a portion of the separator surrounding the through hole is bonded to a peripheral portion of the single cell. The separator having this structure separates a cathode chamber facing a cathode from an anode chamber facing an anode. The first and second interconnectors are members having a flat-plate shape and are disposed so as to face each other in the first direction with the single cell intervening therebetween. The frame member is disposed between the separator and the first interconnector and has a through hole forming the anode chamber. The gas sealing member is disposed between the separator and the second interconnector and has a contact portion which is in contact with both the surface of the separator on the side toward the second interconnector and the surface of the second interconnector on the side toward the separator, thereby sealing the cathode chamber.
In some cases, the electricity generation unit may include a weld portion which is formed by, for example, laser welding and seals between the separator and the frame member. In such a structure, the surface flatness of the separator may fail to be maintained due to the presence of a protrusion (e.g., bead) formed on the weld portion, and the gas sealing member may be prevented from being in good contact with the surface of the separator, resulting in impairment of gas sealing of the cathode chamber by the gas sealing member. In a conventionally known technique, the frame member has a stepped structure including a thin plate portion and a thick plate portion disposed outside of the thin plate portion (i.e., on the side more remote from a peripheral portion of a single cell) in a second direction perpendicular to a first direction, and a separator is welded to the thin plate portion of the frame member, to thereby secure a space that accommodates a protrusion of a weld portion and to prevent impairment of gas sealing of the cathode chamber by the gas sealing member due to the presence of the protrusion (see, for example, Patent Document 1).